The present invention relates to methods of fabricating thermal heads which are used in thermal recording apparatus such as various types of printers, plotters, facsimile machines, recorders and the like. Particularly, the present invention relates to a method of fabricating a thermal head which can enhance efficiency of etching processing to be performed on a lower protective layer using a mask in order to improve adhesion thereof to a carbon protective film, prior to forming the carbon protective layer having an excellent wear resistance.
Thermal recording materials comprising a thermal recording layer on a substrate of a film or the like are commonly used to record, for example, images produced in diagnosis by ultrasonic scanning.
This recording method, also referred to as thermal recording, eliminates the need for wet processing and offers several advantages including convenience in handling. Hence, in recent years, use of the thermal recording is not limited to small-scale applications such as images produced in diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, thermal recording involves the use of a thermal head having a glaze, in which heating elements comprising heat generating resistors (hereinafter referred to as heaters) and electrodes, used for heating the thermal recording layer of the thermal recording material to record an image are arranged in one direction (main scanning direction) and, with the glaze urged at a small pressure against the thermal recording layer of the thermal recording material (hereinafter referred to simply as thermal recording layer), the two members are moved relative to each other in an auxiliary scanning direction perpendicular to the main scanning direction, energy is applied to the heaters of the respective pixels in the glaze in accordance with image data to be recorded which were supplied from an image data supply source such as MRI or CT in order to heat the thermal recording layer thereby accomplishing image reproduction.
A protective film is formed on the surface of the glaze of the thermal head in order to protect the heaters for heating the thermal recording material, the associated electrodes and the like. Therefore, it is this protective film that contacts She thermal recording material during thermal recording and the heaters heat the thermal recording material through this protective film so as to perform thermal recording.
The above-described protective film is usually made of wear-resistant ceramics and the like; however, during thermal recording, the surface of the protective film is heated and kept in sliding contact with the thermal recording material, so it will gradually wear and deteriorate upon repeated recording.
If the resultant wear of the protective film progresses, density unevenness will occur on the thermal image or a desired protective strength can not be maintained and, hence, the ability of the film to protect the heaters and the like is impaired to such an extent that the intended image recording is no longer possible (the head has lost its function).
Particularly in the applications such as the aforementioned medical use which require multiple gradation images of high quality, a trend is toward ensuring a desired high image quality by adopting thermal films with highly rigid substrates such as polyester films and also increasing setting values of recording temperature (energy applied) and of pressure at which the thermal head is urged against the thermal recording material.
Under these circumstances, as compared with a conventional thermal recording system, a greater dynamic stress and more heat are exerted on the protective film of the thermal head, permitting wear and corrosion (or wear due to corrosion) more likely to progress. Further, in a thermal film using a polyester film and the like as the substrate thereof, a substance which causes the corrosion of the protective film such as water contained in the thermal recording layer does not penetrate into the substrates and sticks on the surface of the thermal head, namely, the protective layer thereof so that a concentration of a corrosive substance on the surface of the protective layer is likely to be increased thereby causing a further progress of corrosion.
With a view to preventing the wear of the protective film on such a thermal head and improving its durability, a number of techniques to perform improvement of the protective film, improvement of the thermal recording material, improvement of a recording condition or the like have been proposed or practically executed.
Among the above-described improvements, the improvement of the thermal recording material primarily intends to reduce a quantity of a component which will cause wear or corrosion; however, in this case, an adverse effect such as dust deposition on the head, sticking of the head or the like may occurs at the same time whereupon a sufficient wear resistant effect may not be obtained.
On the other hand, the improvement of the recording condition intends to reduce a maximum temperature or recording pressure of thermal recording, but this method sometimes has an effect on an image quality of the recording image, in particular, in an application in which a high-quality image is required, a sufficient effect can not be obtained in some cases.
Therefore, in order to prevent wear of the protective film on the thermal head, a multiplicity of techniques to enhance performance of the protective film has been studied.
As a method to enhance the wear resistance of the protective film as described above, Unexamined Published Japanese Patent Application No. 62-227763 discloses use of a diamond thin film as a protective film.
Further, it has been proposed that a thermal head having an excellent durability can be realized by enhancing the wear resistance of the protective film by means of provision of a plurality of layers of the protective films.
For example, Unexamined Published Japanese Patent Application (Kokai) No. 7-132628 discloses a thermal head which has a dual protective film comprising a lower silicon-based compound layer and an overlying diamond-like carbon layer (DLC layer, hereinafter also referred to simply as xe2x80x9ccarbon layerxe2x80x9d) whereby the potential wear and breakage of the protective film are significantly reduced to ensure that high-quality images can be recorded over an extended period of time.
When a thermal head having such a dual-layer structure is produced, in order to enhance adhesion between the lower silicon-based compound layer (for example, silicon nitride layer, hereinafter referred to as xe2x80x9csilicon nitride layerxe2x80x9d) and the overlying carbon layer, the overlying carbon layer is formed after a surface of the lower silicon nitride layer is subjected to etching processing by plasma and the like. Namely, smear on the surface of the lower silicon nitride layer is removed by etching processing to have the surface cleaned.
On this occasion, the carbon layer is formed in a limited area such as a portion just above the heater and the like so that, when the surface of the silicon nitride layer is cleaned by the above-described etching processing, the etching processing is performed after other areas than the portion where the above-described carbon layer is formed is shielded with a mask. As an example of the mask, a structure composed of a stainless steel (SUS) material and the like is ordinarily repeatedly used.
After the etching processing is performed, the carbon layer is formed by a method such as sputtering or the like. On this occasion, when the mask composed of the above-described stainless steel material is used, there occurred a problem that the processing was not always stabilized. Namely, in some cases, the carbon layer having good adhesion was able to be formed; in other cases, the formed carbon layer had insufficient adhesion thereby peeling off while in use.
The present invention has been accomplished under these circumstances and has as an object providing a method of fabricating a thermal head which has solved problems of the conventional techniques and is capable of stabilizing a process for forming a carbon layer in order to enhance the adhesion between a lower silicon-based compound layer and the upper carbon layer.
The stated object of the present invention can be attained by a method of fabricating a thermal head, comprising the steps of: forming a lower protective layer comprising ceramics for protecting a plurality of heat-generating resistors and electrodes; subjecting the lower protective layer to etching processing by a plasma; and forming a carbon protective layer on the thus subjected lower protective layer, wherein the etching processing is performed using a mask which defines an area where the carbon protective layer is formed, a protective layer is formed on a surface of the mask, and the protective layer is made of a material which is etched at an extremely slow rate or substantially not etched compared with ceramics composing the lower protective layer and/or which does not impart an adverse effect to the carbon protective layer that is subsequently formed.
Preferably, the protective layer is made of carbon, and the carbon is the material which is etched in the extremely slow rate or substantially not etched compared with the ceramics composing the protective layer and/or which does not impart the adverse effect to the carbon protective layer that is subsequently formed.
Preferably, the mask is made of stainless steel.
In the method of fabricating the thermal head according to the present invention, based on an analysis of causes which unstabilized the process by the above-described conventional techniques, a surface of the stainless steel material used as a mask is covered with a material which is etched in an extremely slow rate or substantially not etched compared with the stainless steel material or another material which does not have an adverse effect on the carbon protective layer that is subsequently formed thereon. By this arrangement, an occurrence of a phenomenon that the stainless steel material which constitutes the mask is etched and a portion of the thus etched-away material is deposited as a foreign matter on the ceramic-based lower protective layer is prevented thereby enhancing adhesion between the carbon protective layer that is subsequently formed and the ceramic-based lower protective layer. As a result, the carbon protective layer can be prevented from peeling off the ceramic-based lower protective layer.
Namely, the above-described causes which unstabilized the process was attributable to that, since there was not much difference between a rate at which the stainless steel material used as a mask was etched by plasma and another rate at which the ceramic-based lower protective layer that was the principal target for etching was etched, the stainless steel material was etched whereby a portion of the resultant etched-off material was deposited on the ceramic-based lower protective layer as a foreign matter.
Therefore, the surface of the stainless steel material composing the mask is covered by a material which is hard to be etched compared with this stainless steel material (that is, etching speed thereof is extremely low compared with that of the ceramic-based lower protective layer which is a principal target for the above-described etching) and does not have an adverse effect on the carbon protective layer that is subsequently formed thereon. Carbon is effective as a covering material but the present invention is not limited thereto.
According to the present invention, by allowing a thermal head to have arrangements as described above, a method which is capable of fabricating the thermal head having a carbon protective layer on a ceramic-based protective layer in a consistent manner can be achieved.